1. Field of the Invention
This invention relates to an image display apparatus, a color signal correction apparatus and a color signal correction method which are used for the image display apparatus, and is one which is preferable when it is used for, in particular, a television receiver, a display apparatus and so on, which receive television signals, image signals for a computer and so on, and which display images by use of an indicator such as a plasma display panel (PDP), an electroluminescence display (ELD), an electron-emission type display and so on.
2. Description of the Related Art
Speaking about such TV signals as in NTSC and HDTV which are image signals by way of example, they are figured out with a receiver which used a CRT as a target, and are outputted after a γ characteristic which the CRT has (non-linear characteristic of a luminance signal—light emission luminance characteristic) has been corrected in advance (called as γ correction).
However, there occurs such a phenomenon that light emission quantity of an indicator does not precisely correspond to a luminance level.
Taking one example, in case of an electron-emission type display, there may be shown such a saturation phenomenon that, when selection time of one pixel is lengthened by adoption of line sequential drive, in a multiple electron beam source which is of simple matrix wiring such as a cold cathode device, as a result of that, such time that a light emitter (phosphor) of one pixel is subject to electron beam irradiation is lengthened too much, and light emission quantity of the phosphor is made not to be in proportion to electron beam irradiation time. In this specification, this phenomenon is called saturation of a phosphor.
A level of this saturation is varied in accordance with a type of a phosphor, density of an electron beam, irradiation time of an electron beam, and so on.
And, when a saturation characteristic is varied in accordance with a material of the phosphor and a type of a color, color balance of an image to be displayed, in particular, a chromaticity point of an achromatic color (white color) does not become a desired value.
A technology for solving the suchlike problem is disclosed in, for example, Patent Document 1 (JP-A-2000-75833 gazette). A saturation characteristic in accordance with gradations of respective phosphors of RGB which were described in the patent document 1 is shown in FIG. 13. In FIG. 13, a horizontal axis indicates standardized luminance data, and a vertical axis indicates standardized luminance.
And, on one shown in FIG. 13, an inverse function (correction function) of a convex curve is calculated, and correction luminance data is calculated by substituting the correction function with input luminance data (also called luminance desired value, gradation data), and thereby, light emission luminance of the florescent body becomes linear to the input luminance data. And, RGB have that correction function, respectively, and thereby, saturation characteristics of respective phosphors can be corrected.
Other than this, an adjustment method of a white color has been proposed in Patent Document 2 (JP-A-63-160492 gazette), Patent Document 3 (JP-A-2001-119717 gazette), and so on.
However, in these conventional technologies, suppression of variation of chromaticity points of an achromatic color and a chromatic color is not sufficient.
For example, in a method for correcting an output luminance level to an input gradation level of a color signal as described in the patent document 1, correction of non-linearity of luminance becomes possible. However, it has not yet possible to suppress such a phenomenon that a chromaticity point of the achromatic color is varied interlocking with change of a gradation level. For details, if a chromaticity point of a white color in respective gradation levels after correction of saturation of phosphors was carried out, is shown on a CIExy chromaticity diagram, there may occur such a case that, interlocking with change of the gradation level, the chromaticity point of the white color is varied from a point 21 to a point 22, as measurement points shown in FIG. 14.
In this connection, an inventor of this invention has been studied with his whole heart with regard to this point, and proceeded with various researches, and as a result of that, learned that it is resulted from such a fact that there exist different saturation characteristics in all of tristimulus values XYZ, in a monochromatic gradation characteristic of phosphors of respective colors.
In particular, most of phosphors which are used in an indicator, as shown in FIG. 15, show such a gradation characteristic that Z in phosphors of red and green is close to a straight line, as compared with X, Y whose saturation characteristics are relatively similar to each other. In addition, in FIG. 15, a horizontal axis indicates standardized gradation data, and a vertical axis indicates standardized tristimulus value.
Therefore, as shown in FIG. 16, if correction is carried out by use of an inverse function of a saturation characteristic of Y as the correction function, even if a gradation characteristic of Y becomes linear, Z becomes over correction, and a gradation characteristic of Z becomes a downward convex characteristic. In addition, in FIG. 16, a horizontal axis represents standardized gradation data, and a vertical axis represents standardized corrected tristimulus value.
From FIG. 16, it is understood that, by such a fact that a Z component falls short relatively in red and green, balance of XYZ after correction, i.e., difference of a value of X or Y and a value of Z is varied in accordance with change of a gradation level, and thereby, a chromaticity point is varied even in a simple color which is a chromatic color. As a matter of course, in a white color of an achromatic color, in some gradation, only a component of Z is extremely reduced, and thereby, balance of XYZ components is disrupted, and therefore, values of x and y which are chromaticity coordinates of a white color are changed, and a chromaticity point of white is varied.